Our invention relates to an oscillator control circuit, and particularly to a circuit for controlling the frequency of a single oscillator that supplies signals to a radio transmitter and to a radio receiver that operate with a radio repeater or similar installation.
In some radio communication systems, a station will transmit on a first frequency and receive on a second frequency. Thus, for example, a mobile radio station on a vehicle might transmit to a base station or repeater on a frequency of 800 MHz and receive on a frequency of 845 MHz. The repeater of course would receive on the frequency of 800 MHz and transmit on the frequency of 845 MHz. In the interests of economy and simplification, the mobile station preferably would have only one stable oscillator which produces a 800 MHz signal for transmitting, and which mixes or heterodynes the 800 MHz signal with the received 845 MHz signal to produce an intermediate frequency of 45 MHz which is detected for utilization. However, in the receiving condition, some amount of the 800 MHz oscillator signal is radiated, so that if the mobile station is close to the base station or repeater, this 800 MHz signal will capture or control the repeater even though the mobile station does not to intend to do so. This of course prevents other mobile stations from using the repeater on that frequency or channel.
In addition, the single oscillator must not or should not be modulated in the receiving condition, as such modulation causes noise or other problems in the receiving condition, and may cause false alerting in a tone controlled system.